The present invention relates to a complex oxide containing titanium oxide obtained by a vapor phase method, a process for producing the complex oxide, and a composition containing the complex oxide. More specifically, the present invention relates to a complex oxide containing titanium oxide produced from titanium tetrachloride and a solution or slurry of a salt containing a metallic element. The present invention also relates to a process for producing the complex metal oxide and a composition containing the complex oxide.
The industrial field where ultrafine particulate titanium oxide (sometimes also called as xe2x80x9ctitanium dioxidexe2x80x9d) is applied has recently expanded. For example, studies are being made on uses over a wide range of fields, such as starting material for high-performance dielectric, ultraviolet-shielding material, additive to silicone rubber and photocatalytic material.
Use of ultrafine particulate titanium oxide as a starting material for a Perovskite-type dielectric is described below.
As shown in the following reaction formula, a titanium oxide is subjected to a solid phase reaction at a temperature of about 1,200xc2x0 C. to produce a barium titanate dielectric material:
Reaction Formula:
BaCO3+TiO2xe2x86x92BaTiO3+CO2 
In this case, barium carbonate decomposes at about 700xc2x0 C. to produce BaO having high ionicity and this BaO diffuses into a TiO2 particle having a covalent bonding property to produce barium titanate. The particle size of barium titanate is determined by the crystal size of TiO2 at the time of reaction and therefore, the crystallinity, the particle size and the like of titanium oxide as a starting material are important.
Also, to cope with the requirement for higher dielectric constant or smaller size of a ceramic capacitor, there is a demand for ultrafine particulate barium titanate and heretofore, investigations have been made for forming ultrafine particles of titanium oxide as a starting material. The ultrafine particle has not been clearly defined so far but, in general, indicates a fine particle having a primary particle size of about 0.1 xcexcm or less.
However, titanium oxide particles having a particle size of 0.1 xcexcm or less undertake vigorous growth in the vicinity of the above-described reaction temperature of 700xc2x0 C., and disadvantageously fail in contributing to the formation of ultrafine particulate barium titanate. Accordingly, titanium oxide suitable for this purpose is being demanded.
In view of the photocatalytic activity, uses for antifouling, sterilization or deodorization purpose, or the ultraviolet-shielding effect, uses of titanium oxide for cosmetic materials are attracting attention. These uses are supported by excellent safety, processability, functionality and durability of titanium oxide.
Photocatalysis is described below.
Titanium oxide has a property of absorbing an ultraviolet ray at a wavelength of about 400 nm or less and exciting an electron. An electron and a hole generated and reaching the particle surface combine with oxygen or water to generate various radicals. The radical seeds mainly show an oxidation activity and decompose by oxidation a substance adhering to the surface. This is a basic principle of photocatalysis.
In order to enhance the catalytic activity, for example, the following methods are used.
(1) Reducing particle size
This is very effective for inhibiting recombination of an electron and a hole generated.
(2) Enhancing crystallinity
This is effective for increasing the diffusion rate of an electron and a hole generated to the surface.
(3) Performing charge separation
An electron and a hole generated are subjected to charge separation and the yield thereof reaching the surface is increased.
(4) Controlling band gap
The band gap is reduced (maximum absorption wavelength is increased) by adding trace impurities, whereby use of light, for example, sunlight can be elevated.
Among the above, various additives are being studied for the purpose of (3) and (4) in recent years.
In order to coat fine particulate titanium oxide having the above-described photocatalytic effect on a support, a method of coating a slurry obtained by mixing it with a binder on a support and then baking the coating at 600xc2x0 C. or more may be used. However, at this time, primary particles grow due to sintering, and as a result, the catalytic activity disadvantageously decreases.
Conventionally, in the production process of fine particulate composite oxide containing titanium oxide, a surface treatment by a liquid phase method is a representative process. However, this process has a problem in that a step of removing a solvent, namely, filtration and drying is necessary. In addition, in the case where the solvent is water, the water works out to a binder and accelerates the coagulation, and therefore, a step of strongly cracking or pulverizing the agglomerate is required. Furthermore, when an organic solvent is employed, a system for recovering the solvent is necessary and a cumbersome production process results.
JP-A-11-228139 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a technique for obtaining a composite powder material by atomizing and combusting an emulsion starting from an organic solvent/dispersant and an aqueous solution having dissolved therein a salt containing titanium oxide of BET 15 m2/g or more and a metallic element. This method is, however, disadvantageous in that the process takes a long time, and a large amount of a flammable solvent is necessary for the combustion.
In view of the above-described uses of fine particulate metal oxide, an object of the present invention is to provide a simple and easy process for producing ultrafine particulate complex oxide (sometimes also called xe2x80x9ccompound oxidexe2x80x9d) containing titanium oxide. Another object of the present invention includes providing the complex oxide and a composition containing the complex oxide.
As a result of extensive investigations, the present inventors have found that the above-described problems can be solved by producing an ultrafine particulate complex oxide having a BET specific surface area of about 5 to about 200 m2/g in a one-stage reaction, and more specifically, in a vapor phase process for producing a complex oxide containing titanium oxide, by reacting a starting material gas containing titanium tetrachloride, an oxidizing gas, each gas being pre-heated at about 700xc2x0 C. or more, and a solution or slurry of a salt containing a metallic element. The term xe2x80x9cmetallic elementxe2x80x9d as used in the present invention includes the elements defined in Table 1.
That is, the present invention comprises the following embodiments.
(1) A process for producing ultrafine particulate complex oxide containing titanium oxide, comprising vapor-phase producing a complex oxide containing titanium oxide having a BET specific surface area of about 5 to about 200 m2/g, wherein a starting material gas containing titanium tetrachloride and an oxidizing gas, each gas being preheated to about 700xc2x0 C. or more, are reacted with a solution or a slurry of a salt containing a metallic element.
(2) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the complex oxide contains a bond of xe2x80x94Tixe2x80x94O-metallic element-.
(3) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the salt containing a metallic element is at least one salt selected from the group consisting of a hydroxide, a halide, a nitrate, a sulfate, a carbonate and an organic acid salt each containing a transition metal element.
(4) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the salt containing a metallic element is at least one salt selected from the group consisting of a hydroxide, a halide, a nitrate, a sulfate, a carbonate and an organic acid salt each containing an alkaline earth metal.
(5) The process for producing a complex oxide containing titanium oxide as described in 1, wherein the salt containing a metallic element is at least one salt selected from the group consisting of a hydroxide, a halide, a nitrate, a sulfate, a carbonate and an organic acid salt each containing an alkali metal.
(6) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the salt containing a metallic element is at least one salt selected from the group consisting of a hydroxide, a halide, a nitrate, a sulfate, a carbonate and an organic acid salt each containing a metal of Group IIIb or IVb.
(7) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the solvent of the solution or slurry has a boiling point of about 40xc2x0 C. or more at an atmospheric pressure.
(8) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the solution or slurry is fed from a portion upstream from than the point of initiating the reaction of the starting material gas containing titanium tetrachloride with the oxidizing gas.
(9) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the solution or slurry is fed between the point of initiating the reaction of the starting material gas containing titanium tetrachloride with the oxidizing gas and the portion about 200 mm downstream from that.
(10) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the solution or slurry is fed in the atomized state.
(11) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the reaction is performed by feeding the starting material gas containing titanium tetrachloride and the oxidizing gas, each gas being preheated at about 700xc2x0 C. or more, to a reaction tube each at a flow rate of about 10 m/sec or more.
(12) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the reaction is performed by allowing the starting material gas containing titanium tetrachloride and the oxidizing gas to stay in the reaction tube under a high-temperature condition such that the temperature within the reaction tube exceeds about 600xc2x0 C., for a time period of about 1 second or less.
(13) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the average flow rate of the starting material gas within the reaction tube is about 5 m/sec or more.
(14) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein a turbulence is generated upon feeding of the starting material gas containing titanium tetrachloride and the oxidizing gas, each gas being preheated, into the reaction tube.
(15) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the starting material gas containing titanium tetrachloride and the oxidizing gas are fed into the reaction tube through a coaxial parallel-flow nozzle and the inner tube of the coaxial parallel-flow nozzle has an inside diameter of about 50 mm or less.
(16) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the starting material gas containing titanium tetrachloride contains from about 10 to 100 vol % of titanium tetrachloride.
(17) The process for producing a complex oxide containing titanium oxide as described in 1 above, wherein the starting material gas containing titanium tetrachloride and the oxidizing gas each is preheated at about 1,000xc2x0 C. or more.
(18) An ultrafine particulate complex oxide containing titanium oxide, which is produced by the production process described in any one of 1 to 17 above.
(19) An ultrafine particulate complex oxide containing titanium oxide, which is an ultrafine particulate complex oxide having a BET specific surface area of about 5 to about 200 m2/g.
(20) An ultrafine particulate complex oxide containing titanium oxide as described in 19 above, which is in the mixed crystal state of allowing a bond of xe2x80x94Tixe2x80x94O-metallic element- to be present within a primary particle.
(21) An ultrafine particulate complex oxide containing titanium oxide as described in 19 above, wherein the average primary particle size is from about 0.008 to about 0.3 xcexcm.
(22) An ultrafine particulate complex oxide containing titanium oxide as described in 19, wherein the decrease percentage of the BET specific surface area after heating at 700xc2x0 C. for 1 hour is about 20% or less.
(23) A composition comprising at least one ultrafine particulate complex oxide containing titanium oxide selected from the ultrafine particulate complex oxides containing titanium oxide described in any one of 18 to 22 above.